Wireless communication devices communicate with one another by transmitting and receiving data to and from one another over wireless channels. A wireless channel may be an electromagnetic frequency, typically radio frequency (RF), or a set or narrow range of such frequencies. Signals transmitted on a wireless channel can be disrupted by interference on that channel. From the perspective of a given pair of devices transmitting and receiving signals to and from one another on a wireless channel, any other signals which disrupt those signals constitute interference. Interference can be created by other wireless communication devices communicating with one another on the same or a nearby channel, or by any other source of electromagnetic radiation which causes such disruption.
Since the advent of wireless radio communications, interference has always been an important issue to deal with to achieve reliable communications. For this reason, in many countries, a large portion of radio frequency bands are licensed and regulated by the relevant government. Any user needs to pay a premium to gain the access to the licensed bands.
In contrast, there also exist a few unlicensed bands, e.g. the industrial, scientific and medical (ISM) bands or the “whitespace” bands previously reserved in some countries for analogue television broadcasts, which are essentially free to use subject to certain regional regulations. In recent years, the use of un-licensed bands is becoming increasingly popular. One of the reasons is low cost. Another reason is that many applications, such as sensor network, wireless lighting control etc., do not need to transmit a lot of data continuously for a long interval of time.
With the increasing popularity and relatively weaker regulation in the unlicensed bands, interference in such bands is also becoming an important issue, exacerbated by the free access nature of unlicensed spectrum and the lack of stringent regulations.
There are mainly two types of interference that affect the radio communication performance. One is in-band interference, coming from all sorts of radio transmitters sharing the same band or transmitter leakage from out-of-band transmitters; the other is the leakage of out-of-band signal power captured by a receiver due to the receiver frontend filter being non-ideal in practice (out-of-band interference).
To deal with interferences, attempts may be made to detect and characterize the interferences so that appropriate strategies can be undertaken. One known type of approaches to try and deal with interference detection is to use an initial channel scanning operation. The initial scanning operation is performed to look for the available channels in the unlicensed bands. This initial channel scanning time is much longer than the typical packet duration. If this initial channel scanning reveals that there is no significant interference, the user starts to use this channel for the desired communication applications. In normal operations, listen-before-talk (LBT) type of approaches can be employed for short-term access of the channel.
There are two main drawbacks of this approach. One is that this approach cannot capture the potentially rapid-changing interference situations in a real-time manner. The other is the basic channel scan misses the important characteristics of the interference scenarios as well as the application relevance.
Specifically, often neither the desired application nor the interference sources require a constant occupancy of the channel, and often exhibit certain duty-cycle behaviours. For many applications in unlicensed spectrum, it is not required to transmit continuously a significant amount of data. Instead, many such applications only require certain burst transmissions, whether random or following a fixed schedule. Consequently, also due to the application characteristics of the potential users, the interference in many cases is not permanent and is of certain duty-cycle behaviours. As another example, in some applications such as RFID (“Radio Frequency Identification”), a frequency hopping scheme is used, instead of LBT.
Such frequency hopping also introduces a duty-cycle type of impact to a user, in the sense that the frequency hopper appears to be an interference source when it hops into the current user channel, and does not appear when it hops away from the current user channel (even though it is still active and may therefore may still be creating interference on other channel(s)).
Thus, the communication performance in the chosen channels cannot be guaranteed in many cases by the interference scanning approach because it cannot capture a fast-changing interference situation. Further, this approach also does not provide sufficient information to provide detailed network situation diagnosis.
US Patent application No. 2008/146156 discloses a method of predictive sensing of periodic intermittent interference which includes the measuring of energy on a channel for an indication of periodic intermittent interference. If a current energy level on the channel is below the new threshold and previous on energy characteristic suggests that the current energy level will remain below the new threshold for a predetermined minimum period of time, a clear channel indication is provided.